1. Field of the Invention
The present invention relates to a pressure detector circuit having a pressure to resistance transducer or sensor, and more particularly, to the circuit suitable for use in an automatic opening/closing apparatus, for example, a power window of an automobile to detect an obstacle between a window frame and a closing window glass thereof.
2. Description of the Prior Art
A power window device mounted on a door of an automobile drives a window glass received in the door by a motor. For example, a controller controls the motor to open the window glass by supplying reverse power to the motor when an obstacle is put or clamped between a window frame and the window glass during closing the window glass. Then, a pressure sensor is mounted or embedded in the door frame to detect the putting or clamping of the obstacle.
FIG. 1 shows the pressure to resistance transducer or sensor 10 that comprises a core metal or electrode 1, a pressure sensitive conductive or resistive rubber 2 coaxially provided or disposed around the core 1, a sheath metal electrode (or a wire braid) 3 disposed around the rubber 2 and a dielectric cover 4 disposed around the sheath 3.
The pressure sensor 10 generally increases exponentially its resistance Rs upon the application of pressure. Assuming the applied pressure to be P, the resistance Rs of pressure sensor 10 is rendered as follows: EQU Rs.varies.KP.sup.-N
where K and N are positive constants, respectfully. The present inventor proposed a basic pressure detector circuit in Japanese Patent Application (JPA) No. 2-198224 filed Jul. 26, 1990 and laid open for public inspection under JP-A-4-83136.
FIG. 2 shows the basic pressure detector circuit proposed by the inventor. It comprises a reference resistor 11 connected between an inverting input and an output of an operational (OP) amplifier 12. The inverting input of amplifier 12 is also grounded through the pressure sensor 10 as shown in FIG. 1. The output of amplifier 12 is connected to an output terminal 13 and an inverting input of a three-state comparator 14. The three-state comparator 14 has its noninverting input applied to a reference voltage from a reference voltage source 15 and its output is looped or fedback to a noninverting input of OP amplifier 12 through a low-pass filter 16. The low-pass filter 16 includes a resistor 17 connected between the output of comparator 14 and the noninverting input of amplifier 12, and a capacitor 18 connected between the noninverting input of amplifier 12 and ground. Therefore, the pressure detector circuit in ordinary condition provides an output voltage from the terminal 13 identical to the reference voltage even if a resistance of unpressured pressure sensor 10 upon manufacture deviates from a mean value and if an ambient temperature of the automobile equipped therewith is changed. When a force is abruptly applied to the pressure sensor 10, the output voltage is increased from the reference potential due to a time constant of the low-pass filter and then slowly returns to the reference potential. Then, sudden pressure on the pressure sensor 10 will be detected by another comparator (not shown) having a threshold voltage higher than the reference voltage.
The present inventor proposed pressure detector circuits in Japanese Patent Application Nos. 3-228389 to 3-228391 each filed on Aug. 13, 1991.
FIG. 3 shows the pressure detector circuit proposed by JPA No. 3-228391 that is similar to that of FIG. 2 except that reference resistor 11 and pressure sensor 10 are connected to ground and the output of amplifier 12, respectively. Additionally, a switch 19 and a voltage follower 20 are provided within the feedback loop.
The switch 19 is controlled by a signal SW to turn off the feedback loop when the window glass is driven upwardly by a motor (not shown). The voltage follower 20 includes another operational amplifier 20 having its inverting input connected to its output and a noninverting input of amplifier 12, and its noninverting input connected to a low-pass filter 16.
JPA No. 3-228389 is similar to that of FIG. 2 except that a switch 19 and a voltage follower 20 are provided within the feedback loop. JPA No. 3-228390 is similar to that of FIG. 2 except that the reference resistor 11 and pressure sensor 10 are connected to ground and the output of amplifier 12, respectively.
A controller (not shown) shuts down the power supply to the motor to stop the window glass moving after detection of an obstacle between a window frame and the window glass when the obstacle is put therebetween upon closing the window glass. A pressure sensor is used to detect the obstacle and comprises a conductive rubber or coating sensitive to pressure.
However, the above conventional pressure detector circuit cannot detect break and short circuit conditions of the pressure sensor 10 which is less reliable than other components such as resistors, capacitors and OP amps.
In the circuit shown in FIG. 3, the input voltage of voltage follower 20 will be changed upon turning off the switch 19 by a dielectric characteristic of a printed circuit board by ambient moisture or a drift of OP amps. The input voltage is afraid to be decreased slightly by a leakage current through the printed circuit board upon moving the window glass upwardly to stop the window glass by detecting an erroneous output voltage more than the predetermined threshold voltage. In contrast, The input voltage will be increased slightly by another leakage current to a positive supply through the printed circuit board to degrade sensitivity of pressure sensor 10.